Combustion of substances to enable the inhalation of volatile materials contained therein has been practiced for millennia. In more recent times, the health effects of this practice have been extensively studied from an epidemiological viewpoint and the hazards of smoking tobacco have been well documented. Combustion of tobacco, cannabis or other "smoking materials," is accompanied by oxidation, hydrogenation, cracking, distillation and sublimation. The first three of these processes result in the formation of chemical compounds not present in the original source material and it is these products of combustion and pyrolysis that are generally recognized as the most hazardous aspect of smoking. By heating a substance such that distillation and sublimation occur without combustion, only those compounds present in the source material which are sufficiently volatile to boil or sublime at a given temperature will be available for inhalation. By eliminating combustion as a heat source, the health risks and benefits of the volatile compounds present in a source material can be evaluated on their own merits. For example, a study performed by the National Toxicology Program and overseen by the U.S. Food and Drug Administration and the National Cancer Institute concluded that the active principal of cannabis, tetrahydrocannabinol, does not cause cancer and may have protected laboratory animals against malignancies. Nicotine, the active principal of tobacco, is highly toxic and is considered addictive, but it is not carcinogenic. Whether these volatile compounds and others ought to be administered to humans is a question that could be better addressed if a simple means existed for vaporizing the compounds in the absence of combustion.
The advantages of volatilization and inhalation as a drug delivery method include: simplicity, selective extraction of bioactive compounds from crude plant sources and the rapid uptake of substances by the lungs. This rapid uptake leads in turn to bloodstream levels of bioactive substances quickly reaching effective concentrations. The rapidity of action is very desirable to a patient who is seeking relief from symptoms whose onset is sudden and cannot be anticipated. Compared with oral administration, relatively smaller doses can be administered, having a shorter duration of action and enabling the patient to "titrate" the dosage over time. Titrating the dosage can minimize total dosage, thereby reducing the probability of undesirable side effects. These advantages apply to both pure compounds and crude mixtures of compounds.
U.S. Pat. Nos. 4,141,368 and 4,303,083 describe electrical devices for volatilizing desired components of smoking materials without combustion. The first uses an incandescent light bulb as a heating source while the second uses a rheostat or thermostat controlling an electrical resistance heating element. While the second offers the possibility of fine tuning the operating temperature, the adjustment would have to be made repeatedly to compensate for variations in one or more of ambient temperature, rate of inhalation, and voltage in the power source. Since each adjustment requires trial and error, overshooting would lead to the undesirable effect of incomplete combustion. Also, both devices require continuous heating of the source material which generates vapors whether or not inhalation is occurring, thereby both wasting the source material and making accurate dosage difficult. Thus, while recognizing the advantages of avoiding combustion, the devices do not provide means for precise and reproducible temperature control that is required to achieve volatilization without combustion.
U.S. Pat. No. 4,735,217 avoids waste of source material by providing an on/off switch that can be controlled by the user to switch the power off when inhalation is not occurring. However there is no temperature adjustment capability and the principle of switching the power on and off can only be effective with a low mass heating element which makes reproducible temperature control difficult to obtain.
U.S. Pat. Nos. 5,249,586 and 5,388,594 describe electrical heating devices to vaporize tobacco flavor substances contained in artificial cigarettes. The devices are not intended to cause combustion but no means to accurately control temperature are provided. U.S. Pat. No. 5,060,671 falls into the same general category and discloses self-contained electrically heated "smoking devices." The only temperature control is obtained by controlling the amount of time that the heater is energized. The "flavor medium" should be heated to a temperature of 100 to 600.degree. C. and, preferably, 300 to 400.degree. C. U.S. Pat. No. 5,224,498 describes a heating element having a predetermined electrical resistance which is intended to control the temperature of operation of the above devices. Intended operating temperatures are 100 to 600.degree. C., preferably 250 to 500.degree. C. U.S. Pat. No. 5,372,148 teaches a simple electronic controller for use in the above "smoking articles." While the controller delivers a measured amount of energy to a heating element, it contains no temperature sensor or temperature control means, thereby resulting in temperature variation depending upon ambient conditions.
U.S. Pat. No. 5,564,442 teaches that a charge of tobacco in a vaporizer device is to be brought to combustion temperature, thus the device does not avoid the hazards caused by combustion.
Several devices have been suggested to utilize combustion as a heat source, while isolating the material to be vaporized from the fuel material. U.S. Pat. No. 4,219,032 discloses a device resembling a standard tobacco pipe but adding a separate chamber containing e.g. charcoal fitted above the bowl to supply heat to the "smokeable substance." The device also includes a reservoir that may be partially filled with liquid to cool the vapors. U.S. Pat. No. 5,105,831 is a more recent example of this approach and features a carbonaceous fuel element and an "aerosol forming substance" packaged together in a form resembling a cigarette. The "aerosol forming substance" is held in a heat conductive container such that heat from the fuel source reaches it by conduction. Carbon monoxide is generated by the carbonaceous fuel and temperatures near the fuel reach 400 to 600.degree. C.
Some other patents that use an isolated combustion source to generate vapors and/or aerosols include: U.S. Pat. Nos. 4,340,072, 4,474,191, 5,042,509, 5,099,861, 5,105,831, 5,156,170 and 5,345,951.
U.S. Pat. Nos. 4,922,901, 4,947,874 and 4,947,875, describe drug delivery, smoking, and flavor delivery articles comprising a reusable controller coupled with a disposable heating element. The heating element, having a specific surface area greater than 1.0 m.sup.2 /g, is impregnated with an aerosol forming material. The temperature control is time based or current modulation. No means of temperature sensing is described. The preferred temperature range given is 150 to 350.degree. C., not to exceed 550.degree. C. U.S. Pat. No. 5,388,574 discloses an alternative means of temperature control based upon the use of sensors or thermostats such as bimetallic strips.
U.S. Pat. No. 4,907,606 describes specially modified tobacco compositions and devices intended to heat the compositions and liberate nicotine by electricity, a gas burner, or by the mixing of liquids to liberate heat. The electrically heated version of the device includes a temperature sensor controlled by on/off switching of current. The device is intended for use with chemically modified tobacco which is capable of releasing nicotine at a relatively low temperature of about 30 to 200.degree. C.
U.S. Pat. No. 5,388,574 is another example of an aerosol delivery article that is limited in applicability to specific formulations. The devices of this patent incorporate a first nebulizing stage and a second heating stage. Multi-component aerosol forming materials are introduced into an ultrasound generator, i.e. a nebulizer, which disperses them into relatively large particles 5 to 50 .mu.m in diameter. In the second stage, the dispersion is heated to a temperature below that which would vaporize the active ingredients, but which vaporizes or otherwise activates the aerosol generating ingredient (s). Thus submicron particles are generated without evaporating and subsequently condensing the active ingredient(s). The aerosol is subjected to temperatures in the heating stage of from 50 to 400.degree. C. Surface temperatures in the heating stage are from 200 to 600.degree. C., preferably from 200 to 300.degree. C.
While the prior art has proposed devices for the purpose of vaporizing substances in the absence of combustion, the present invention provides more precise and reproducible control of temperature than in the prior art. In addition, the invention provides a convenient means of controlling the time of exposure of the source material to elevated temperatures. These advantages are especially important when the compounds to be delivered by vaporization offer little margin for error between the temperature of vaporization and the temperature at which thermal degradation occurs.
Accordingly, it is an object of the present invention to produce a device and method which can accomplish vaporization of a volatile compound to make such compound available for inhalation without generating toxic or carcinogenic substances that are by-products of combustion and pyrolysis.
It is a further object to deliver controlled amounts of bio-active or flavor compounds to an individual through inhalation.
More particularly, it is an object of this invention to utilize vaporization of a volatile source material in the absence of combustion to provide an inhalation delivery system combining efficient usage of source material, accurate delivery dosage, and minimum emission of vapors into the ambient air.
It is a still further object to produce a vapor delivery system having enhanced temperature stability.
These and still further objects are described in the ensuing detailed description of the invention.